Donnelly was born in Dublin, the son of a master plumber.[1] He was the sixth of eight surviving children. The 1911 Census lists him living with his family at 34 Wexford Street.[1] Apprenticed as a plumber, he became involved in the Irish Volunteers. During the Easter Rising of 1916 he was the commander of C Company of Éamon de Valera's command in Boland's Mill.

Donnelly took the anti-Treaty side in the Civil War. He founded the National Association of the Old IRA in an attempt to mend some of the rifts in the Republican Movement. He was a member of the provisional National Executive of the Republican Prisoners' Release Association.[3] On 2 March 1940, he was one of the founders of Córas na Poblachta and served as its president.[4] He became a founding member of Clann na Poblachta in 1946.[4]

As one of the most senior surviving veterans of the Rising, he played a prominent role in the 50th anniversary commemorations in 1966. He died in December of that year.[5]